Electronic components, such as semiconductor devices, are frequently tested, sometimes multiple times during their manufacture, using automatic test equipment. To perform these tests, automated test equipment may include instruments that generate or measure test signals such that a range of operating conditions can be tested on a particular device. An instrument may generate or measure a pattern of digital signals to enable testing of digital logic within a semiconductor device. Other instruments may generate high frequency analog signals, while others might generate waveforms of arbitrary shapes. Some instruments may also generate or measure relatively high voltages, either for testing high voltage portions of a device or for supplying power to a device under test.
To support testing of multiple types of devices, or to support running multiple tests on the same type of device, automatic test system may be configurable so that different instruments may be coupled to different test points at different times. A test system may include a switch matrix that allows any of a number of instruments to be switched to any of a number of test points.
For low voltage signals, whether analog or digital, the switch matrix may be implemented with reed relays. Reed relays are electromechanical relays, which provide a low on resistance and a low leakage current when off. For higher voltages, electromechanical relays may also be used. However, switching of higher voltage signals can create reliability problems in electromechanical relays that do not occur with lower voltage signals. As one example, electromechanical relays switching high voltages may “stick,” such that they do not open reliably after some period of operation.
The contact resistance of electromechanical relays used for switching high voltages may also vary unpredictably. A variation in contact resistance can be particularly problematic for a test system because a change of even one Ohm in contact resistance can change test results. For example, a relay might be designed for a nominal contact resistance on the order of 50 milliOhms. A change of one Ohm would be significant as a percentage of the expected on resistance. In an automatic test system, a relay might be switched into a different configuration multiple times during the test of a single device, and might test thousands of devices in a day. As a result, if a relay fails after switching even tens of thousands of times, the relay will fail after a very short period of use.
In the past, mercury wettable switches were sometimes used to reduce reliability concerns for higher voltage signals. These switches contained mercury, which could be used to quickly and reliably close the relay without arcing. Mercury wetted switches have significantly longer lives than dry-switched relays. Though, mercury switches recently have become disfavored because they are not suited for modern test systems that are designed to allow a test head to be placed in multiple orientations. Because a mercury switch is position sensitive, if the test head is moved to a position in which the switch does not operate, the test system might not perform as expected. In addition, mercury switches may be prohibited in some locations because of environmental concerns.
Accordingly, some test systems use “dry” electromechanical relays even for high voltage signals. Owners of such test systems may be advised not to reconfigure the test system so as to cause a relay to switch while a high voltage is across it. Such switching, sometimes called “hot switching,” can contribute to the failure of relays after a relatively low number of cycles.
Hot switching can be useful in automatic test systems and other applications. For testing semiconductor devices, throughput of the test system is a significant contributor to efficiency in a manufacturing facility. Hot switching allows the test system to quickly move from stage to stage in a test job, improving throughput. To address reliability concerns with hot switching, some users of automatic test instruments have built test system interface boards that include relatively large relays that are less susceptible damage if hot switched. In addition, these relays may be mounted in sockets such that, if a failure occurs, the relay can be easily replaced.
Alternatives to using electromechanical switches include using solid state relays, such as optoelectronic relays. However, solid state relays have a higher on resistance than reed relays. For some applications, including automatic test systems, a high on resistance is undesirable. In addition, solid state switches can generate crosstalk, which can interfere with testing operations.
A further alternative for avoiding the need for hot switching is to configure the test system to avoid the need for switching high voltage instruments. A sufficient number of high voltage instruments could be provided such that each test point to receive a high voltage signal could have a dedicated connection to an instrument. Though, this approach can be expensive.